A large percentage of aviation accidents result from loss of control due to an aircraft stall. A stall occurs when an aircraft experiences a sudden decrease in lift because the aircraft's “angle of attack” exceeds a critical angle. The aircraft may depart controlled flight, lose altitude, and may crash if there is insufficient altitude to recover.
A spin may develop when the aircraft is in a stalled condition. A spin occurs when an aircraft is stalling, but there is also excessive side slip which causes one wing to stall more deeply than the other, thereby creating more drag on one side of the aircraft and creating a rotation about the longitudinal axis of the aircraft.
Stalls and spins can be avoided by flying an aircraft below its critical angle of attack. To help pilots avoid stall conditions, some aircraft include an Angle of Attack (“AoA”) display or indicator. These displays inform the pilot of their current angle of attack relative to the critical angle, thus providing the pilot awareness of available angle of attack and allowing the pilot proactive control over stall prevention.
In commercial aircraft (so called “Part 25” aircraft), advanced technical solutions may provide automated flight controls designed to prevent stall conditions. For cost and weight reasons, smaller aircraft (so called “Part 23” aircraft) may have less advanced safety measures. The majority of aviation accidents involve smaller aircraft.
Traditionally, pilots of smaller aircraft avoid stalling by monitoring an airspeed indicator. In most cases, audible and/or visual alarms alert pilots to an impending stall. For example, an audible alarm may sound when an aircraft's angle of attack approaches the critical angle. However there usually isn't an AoA indicator to provide rate and trend information prior to the pilot receiving the warning.
While the airspeed indicator is the primary means of avoiding a stall, the airspeed at which the aircraft stalls changes with the aircraft's weight and load factor. Angle of attack is independent of aircraft weight or load factor.
The traditional safety measures to avoid a stall or stall/spin, however, have limited efficacy. For example, a pilot relying on traditional safety measures must quickly ascertain the necessary corrective control-inputs and execute those maneuvers in time to avoid a stall. Unfortunately, traditional solutions fail when a pilot is unable to correctly ascertain the necessary corrective control-inputs due to the pilot's inexperience, stress, or disorientation.